This invention relates generally to an apparatus for measuring blood flow by non-invasive measurement of magnetic susceptibility changes caused either by movement of blood or by variation in the magnetic susceptibility of the blood.
In the treatment of heart diseases it is important to determine the overall effectiveness of the heart as a pump, and to detect and quantify pathological conditions such as ventricular hypertrophy, stenotic or insufficient valves, and intra-cardiac shunts. The rate of cardiac volume change during contraction of the heart (systole) is related to the ventricular ejection velocity and can be used as a measure of myocardial contractility.
The prior art in cardiac output measurements can be divided into three general classes: invasive techniques requiring catheterization; invasive techniques not requiring catheterization; and non-invasive techniques.
In the invasive techniques requiring catheterization, a catheter is passed through an externally accessible vein or artery into the right or left chambers, respectively, of the human heart. Trans-septal venous catheterizations also provide access to the left chambers of the heart. In addition to being used to measure pressure and to withdraw blood samples from the heart, cardiac catheters may be used for measuring cardiac output.
One technique for measuring cardiac output relies upon the fact that the blood has a slight conductivity. As is known, when a conducting fluid moves in a direction perpendicular to the magnetic field an electromotive force is induced perpendicular to both the magnetic field and the direction of flow. This principle has been applied to catheter flowmeters including a self-contained catheter tip device and a catheter applied pair of sensing electrodes used in conjunction with an externally applied magnetic field. Induced voltages give an indication of the blood flow. The induced voltage depends upon the blood vessel diameter and the velocity and it is not possible to determine cardiac flow without knowledge of the vessel diameter.
Invasive techniques not requiring cathterization include fluorescence excitation in which a material is injected into the blood stream and the concentration of the material in the blood is periodically determined at different locations. Super-paramagnetic fluid tracers have also been used wherein the patient is injected with a super-paramagnetic fluid and the concentration of the super-paramagnetic tracer is determined magnetically.
Among the non-invasive techniques are pulse echo ultra-sound, Doppler ultra-sound, pulse pressure measurements, ballistocardiography and impedance plethysmography. In the latter, changes in total electrical thoracic impedance are measured by placing driving and sensing electrodes in relation to the heart-lung-diaphragm system. The measurement depends particularly upon electrode position, the current distribution through the thorax, the frequency of the driving signal, the rheologic properties of blood, and the fluid content of the lungs. Conventional impedance plethysmography has not been widely used for clinical measurements of cardiac output because of the major uncertainties in determining the above factors.
Another method using the same type of equipment is to make impedance measurements with an induction plethysmograph. This instrument operates on the principle that the magnitude of the eddy currents induced in a sample is proportional to the conductivity of the sample. This technique is subject to major limitations and has not yet been successfully applied.
In this method of induction plethysmography a transmitter coil is energized by sinusoidal current which creates a time varying magnetic field in its vicinity. A second coil located a fixed distance from the exciting coil acts as a receiver and the EMF induced in it is measured. An out-of-phase EMF is induced by the conductivity or eddy currents. In addition, an in-phase EMF is also induced in the receiver coil directly from the transmitter coil. This direct coupled or transformer voltage must be reduced to as low a value as possible by minimizing the mutual inductance and capacitance between the transmitter and receiver coils. A phase-sensitive detector is utilized to ensure that only the out-of-phase conductivity signal is observed. The magnitude of the transformer component precludes observation of the in-phase susceptibility signal. Thoracic conductivity measurement using such an instrument might also be applied to cardiac output determinations. This technique has two serious limitations when compared to susceptibility measurements in accordance with the present invention. The magnitude of the susceptibility related signal is proportional to the volume of the heart, whereas the magnitude of the conductivity related signal is proportional to the five thirds power of the volume. This implies that for a given stroke volume a larger conductivity signal will be produced by a larger heart. Thus, it would be difficult to use the conductivity signal for absolute measurements of cardiac output. In addition, such a measurement would be affected by the anisotropy of the conductivity of cardiac muscles and the variations of blood conductivity with hemocrit.